Malignant Mesothelioma (MM) is a relatively common, rapidly progressive and treatment-resistant malignancy linked to asbestos exposure. Both human MM patients and mouse models of this devastating disease show frequent loss of the neurofibromatosis type II tumor suppressor gene, Nf2, and re-expression of Nf2 in Nf2-null MM cells inhibits their proliferation/viability and restrains their motility, implying that Nf2 los plays a fundamental role in MM pathogenesis. Previously, we and others have implicated Merlin, the protein product of the Nf2 gene, in the regulation of p21-activated kinase (Pak) and mammalian sterile twenty-like (Mst) signaling. These two STE20 kinases play an important role in regulating cell proliferation, survival, adhesion, motility, spreading and invasiveness ? properties connected with the malignant phenotype of Nf2-deficient MM cells. We postulate that both pathways are essential for MM tumorigenesis associated with NF2 loss, and that both pathways can be exploited for therapeutic benefit. We propose two aims. In Aim 1 we will determine the response of a Nf2-null MM mouse model to potent, newly developed preclinical Pak small molecule inhibitors. As we expect that aggressive cancers such as MM will adapt and eventually evade single targeted agents such as Pak inhibitors, we also propose to use a newly developed technology to globally measure the activity of all protein kinases prior to and during treatment with anti-Pak agents, with the goal of identifying potential secondary drug targets for combination therapy. In Aim 2 we plan to delineate the role of the Mst, the defining element in the Hippo tumor suppressor pathway, in Merlin- related signaling and pathology in vivo. Using cell-based assays, we will determine the mechanism by which Merlin controls Mst activity, focusing on our recent observation that loss of Merlin induces a switch from active Mst1/Mst1 and Mst2/Mst2 homodimers to inactive Mst1/Mst2 heterodimers. We will also assess the suitability of targeting the Hippo pathway in MM by crossing our MM mouse model into a conditional Yap1-null strain to assess the effects on Yap loss on tumor incidence and progression. The proposed studies will not only increase our understanding of cardinal oncogenic signaling pathways, but could establish Pak and Mst kinases as suitable targets for therapeutic intervention in this otherwise untreatable disease.